680 Folsom Street is one of San Francisco’s most successful redevelopment projects, one that had already been 85 percent leased before construction started in late 2011. To attract coveted tech clients to the South-of-Market location, developer TMG Partners sought to upgrade the 12-story, concrete-clad, asbestos-filled “Class C” office building to 14 stories with a “Class A” rating. Successfully doing so necessitated a complete overhaul—upgrading the structure’s seismic performance, replacing the drab precast skin with a sleek glass curtain wall, and expanding the space vertically and horizontally. This ambitious project, which first began in 2007, took shape against the backdrop of the Great Recession, which forced a hold in 2008 and a value-engineering redesign in 2010.

The original construction employed bolted flange-plate moment connections at every girder-column connection, creating a redundant and ductile space frame. However, given the addition of two floors, this system alone could not provide sufficient strength and deflection capacity to meet the drift requirements of the current code. Moreover, a review of the original drawings revealed a potential global weakness in the lateral system caused by a proportioning problem at the ground floor of the building: the perimeter columns spanned two floors, creating a weak-story mechanism between the ground and third floors.

The design solution was founded on the shinbashira, or central pillar, of the centuries’ old Japanese pagoda. In that architectural tradition, dating as far back as the seventh century, an entire tree trunk forms the shinbashira. The trunk rests in a shallow stone well, and wooden brackets loosely connect the pagoda’s floors to the trunk. During a large earthquake, the shinbashira—acting as a mode shaper—pivots in the stone well, while the loosely connected floors shimmy independently of each other. This mechanism allows seismic forces to dissipate along the full height of the structure.

Steve Tipping invented a modern shinbashira in the form of a concrete core-wall system resting on a single friction-pendulum slider bearing that pivots within its base. Harnessing the strength of the existing moment frame, the system acts as a mode-shaping spine that improves the drift pattern and dictates yielding throughout the frame’s height, thus redistributing seismic deformations throughout the structure and preventing the formation of a weak-story mechanism. In addition, horizontally placed buckling restrained brace (BRB) elements—analogous to the wooden brackets that loosely join a pagoda’s floors to the shinbashira—link the poured concrete collectors to the core.

This innovative design solution benefited the project in several ways:
-- enhanced seismic performance—building performance was tuned to match the 1.7 percent (DBE) drift capacity of the new curtain wall;
-- lowered construction costs, with 10 weeks chopped off the 100-week construction schedule;
-- lowered earthquake-insurance premiums;
-- a more spacious architectural floor plan; and
-- $4 million savings on a $110 million project.